When I was an undergrad, I saw Usborne's Essential Spanish (slim book and cassette) with phrases I thought would be really useful for study abroad. That was some 10 years ago, so there might be something newer and equally useful on the market. The company that puts out the small gold dictionaries in all languages also has a phrase book that, as I recall, has phrases for going to the doctor's office. But maybe that's overkill if you're only going on a tourist vacation of a week or two.
Helpful tip on pronunciation for your crash course in Spanish:
When you see the letter "a" in English, the sound could be any of the following: apple, ape, any, ago, etc. Spanish is much less confusing to pronounce, because it has only 5 vowel sounds. Easier to read than English once you get the hang of which letter unfailingly goes with which sound. To help you learn them, I've paired the Spanish vowels with English words that have the same letter-sound combination:
a = father, e = egg, i = pink ("ee" in my English), o = go, u = lunar
So when you see the name "Irene" in Spanish, you know that "i" is like in "pink" and "e" is like in "egg," so it's similar to ee-ray-nay.
Here's some Costa Rican slang: tico/tica is a Costa Rican guy/female, and "pura vida" (pure life) is what they say when they like something.
The Costa Ricans I've met (in the States) are very friendly, fun-loving people. You're going to have a great time!
juliee